An inverter is used as an apparatus that applies AC voltage to a motor. The inverter converts inputted DC voltage into AC voltage, and outputs the AC voltage to the motor. The inverter described above is controlled based upon a comparison between a carrier and a command value. The command value is a command value involved with the output voltage of the inverter. A first command value V* is firstly generated based upon a rotating positional angle of the motor or a speed command. A second command value V** that is generated based upon the first command value V* is used for the comparison to the carrier. The second command value V** takes a fixed value for each predetermined periodic time (e.g., a periodic time of the carrier).
When a phase voltage with a rectangular wave is outputted from the inverter described above, the command value V* is a rectangular wave and has a periodic time same as the periodic time of the phase voltage. On the other hand, since the command value V* does not always have a fixed value for each predetermined periodic time, the command value V* is updated every predetermined periodic time to generate the command value V** that should be compared to the carrier. For example, FIG. 14 illustrates the periodic time of the carrier by a broken line, and as illustrated in this figure, the command value V* on each start of the periodic time is used as the command value V** for each periodic time of the carrier.
The inverter is controlled based upon the comparison between the command value V** and the carrier illustrated in FIG. 14, so that the inverter outputs the phase voltage V. In the phase voltage V described above, the period in which the phase voltage V takes the maximum value and the period in which the phase voltage takes the minimum value are different to each other. In other words, the phase voltage V has unbalance. A so-called offset is generated in a phase current outputted from the inverter due to the difference described above. In other words, the average in one periodic time of the phase current does not become zero.
As means for solving the problem described above, the technique described in Japanese Patent No. 4205157 can be employed. In Japanese Patent No. 4205157, when the balance of the output voltage is destroyed, the periodic time of the carrier is synchronized with the command value V*.
Japanese Patent Application Laid-Open No. 9-308256 is described as a technique involved with the present invention.